Originally came from Korean. The Korans use the &#12619(won't show up here); symbol to represent laughter. In StarCraft, using the Korean writing system isn't allowed. So, unfortunately for these poor koreans, they had to express laughter somehow.

&#12619(won't show up); is the hard k, (when pronouncing). It ended up being romanized to "kekekekekekee"

It's the equivilant to "hahhaha" or, "jajaja".

It's also commonly used by men who pretend to be cute Japanese children on the internet